Conventionally, compact, slim image-taking devices are incorporated in portable terminals such as cellular phones and PDAs (personal digital assistants), and this makes it possible to transfer not only audible but also visual information to and from a remote location. Image-sensing elements used in such image-taking devices include CCD (charge-coupled device)-type image sensors and CMOS (complementary metal-oxide semiconductor)-type image sensors. Today, image-sensing elements boast of increased numbers of pixels with accordingly high definition, bringing portable terminals offering high-quality images into the market. Typically, image-taking devices incorporated in portable terminals employ an image-taking lens system composed of two or three lens elements. However, while image-taking lens systems employed in image-taking devices provided with a high-definition image-sensing element are expected to offer high resolution, image-taking lens systems composed of two or three lens elements do not provide satisfactory lens performance. This has led to proposals of image-taking lens systems composed of four lens elements, which can achieve still higher performance than those composed of two or three lens elements.
As an image-taking lens system composed of four lens elements, for example, Patent Document 1 listed below discloses a so-called inverted-Ernostar type image-taking lens system composed of, from the object side, a first lens element having a positive refractive power, a second lens element having a negative refractive power, a third lens element having a positive refractive power, and a fourth lens element having a positive refractive power.
Inconveniently, however, according to the conventional technology mentioned above, owing to the inverted-Ernostar type lens construction, the fourth lens element is a positive lens element, and this, as compared with where the fourth lens element is a lens element having a negative refractive power as in a telephoto type construction, results in the image-taking lens system having a principal point closer to the image side, and hence having a longer back focal length. This makes it difficult to shorten the total length of the image-taking lens system. Moreover, of the four lens elements of which the image-taking lens system is composed, only one lens element has a negative refractive power, and this makes it difficult to correct the Petzval sum. Thus, satisfactory performance cannot be obtained in a peripheral part of the image field.
As another image-taking lens system composed of four lens elements, for example, Patent Document 2 listed below discloses a so-called telephoto type image-taking lens system composed of, from the object side, a first lens element having a positive refractive power, a second lens element having a negative refractive power, a third lens element having a positive refractive power, and a fourth lens element having a negative refractive power. This construction gives the image-taking device a shorter total length, and thus makes it compact.
Inconveniently, however, according to Patent Document 2 mentioned above, the image-taking view angle is small, and aberration correction is insufficient; moreover, with that construction, giving the image-taking lens system a shorter total length degrades the lens performance, and this makes it difficult to cope with image-sensing elements with increased number of pixels.
As yet another image-taking lens system composed of four lens elements, for example, Patent Document 3 listed below discloses a so-called telephoto type image-taking lens system composed of, from the object side, a first lens element having a positive refractive power, a second lens element having a negative refractive power, a third lens element having a positive refractive power, and a fourth lens element having a negative refractive power and having a meniscus shape with a concave surface facing the image side.
Typically, between the fourth lens element, which is the most image-side lens element, and the image-sensing element, there is disposed a parallel-plane plate such as an optical low-pass filter, an infrared cut filter, and the seal glass serving as the package of the image-sensing element. Inconveniently, however, according to Patent Document 3 mentioned above, a peripheral part of the fourth lens element with respect to the optical axis considerably protrudes toward the image plane side. Thus, preventing the fourth lens element from interfering with the parallel-plane plate such as the optical low-pass filter requires a long back focal length. A long back focal length spoils the advantage of a telephoto type construction, making it difficult to shorten the total length of the image-taking lens system. Moreover, according to the conventional technology mentioned above, aberration correction is insufficient to cope with image-sensing elements with increased numbers of pixels.